Various derivatives of organic acids have been proposed as plant growth regulators. For example, West German Patent 19 16 054 discloses the use of alpha-hydroxy- or alpha-ketoalkanoic acids, having 7 to 10 carbon atoms, and their derivatives, particularly amides, for promoting the growth of plants under drought conditions. U.S. Pat. No. 3,148,049 discloses certain halogenated keto acids, such as halogenated acetoacetic acid, as plant growth regulators. U.S. Pat. No. 3,351,653 discloses the use of fluorinated hydroxy acids and esters as herbicides. In 1970, Mikami, et al, Agr. Biol. Chem., 34, 977-979, reported test results of a number of hydroxy acids as plant growth regulators. Several of these, particularly, certain aromatic hydroxy acids, were shown to be root growth promoters. However, some of the simple acids, such as glycolic acid, caused suppression of root growth rather than root growth promotion. None of the hydroxy acids revealed any activity in the straight growth-promotion test used.
We have now discovered, to our surprise, that certain sulfur-containing organic acids do act as growth promoters and can increase chlorophyll concentration when applied to growing plants.